The CFD code KIVA has been applied to the simulation of the transient air-assisted fuel injection(AAFI) process, in which air and fuel at moderate pressures are mixed in an interior chamber of the injector before passing through a pintle valve into air at near ambient pressure in a cylinder. On passage through the pintle valve fuel is atomised.Because of the small dimensions of the flow passages within the injector, a very fine computational grid structure is used to accurately resolve the flow behaviour. Adopting an axisymmetric grid structure enables symmetry to be exploited.The computational results are validated with experimental data for fuel jet penetration and spread with time, obtained using Schlieren visualisation.The simulation of air blast atomisation in an engine cannot utilise the fine grid structure above because of the large computational resources required. To overcome this problem, the atomisation process at the annular pintle is modelled using fuel injection capabilities within the KIVA and KIVA-II codes. By maintaining mass and momentum conservation at the pintle exit, between the original KIVA prescription and that described above, an approximation to the accurately modelled injection process is obtained. This enables a relatively coarse three dimensional Cartesian grid system used in modelling an engine cylinder to be employed.The agreement between modelled and experimental results are quite acceptable.